Currently used elastic hinges are designed to allow a stable closed position of temples that are folded adjacent to the front of eyeglasses and a stable position for use of the temples arranged at right angles to the front.
Moreover, such elastic hinges must allow a certain overtravel angle for the temples.
Said elastic hinges are of various kinds. One more widely used type is constituted by an elongated box-like body that is provided with a receptacle inside which an elastic element, such as for example a cylindrical helical spring, and a first hinge articulation element, associated with the elastic element, are inserted during an assembly step that follows the welding of the box-like body to the corresponding temple.
The elastic element is retained by a contrast element that is locked within the receptacle.
The first articulation element is arranged so that a guiding portion thereof can slide within the receptacle and protrudes from it through a front opening with its own hinge head.
The first hinge articulation element is further pivoted with said hinge head to a second hinge articulation element, which is rigidly coupled to the front of the frame.
Said second articulation element is of the cam type in order to allow the stable closed and active positions.
The internal parts of this type of hinge are inserted after welding the hollow box-like body to the temple.
This entails the problem that the hinge manufacturer cannot supply it to the user, i.e., the eyeglasses manufacturer, in the assembled condition.
If this hinge were welded already during assembly, the heat that is generated during welding, performed with conventional technologies, and diffuses uniformly throughout the box-like body and its contents, would damage irreparably in particular the spring, which by reannealing would lose its elasticity characteristic.
Other types of hinge that are electrowelded are also known.
Electrowelding, in addition to be impulsive, i.e., lasting for an extremely short time, allows to generate limited heat and to localize said heat only in certain preset points.
These hinges can be welded even when they are assembled and can therefore be supplied in a preassembled condition by the hinge manufacturer to the user.
Hinges of this type are known for example from DE-19856117 or from EP-0679920.
In the first of these two patents, the elastic articulation mechanism of the hinge is inserted in the front hole of a closed box and is retained inside by a guiding cylinder, which is locked by a stud provided on the box.
The second of these two patents describes a box that is open on the bottom, and the problem that arises is that the internal mechanism may escape when the hinge is supplied loose by the manufacturer to the user.
The solution that has been adopted is to create inside the box a shoulder on which the spring of the mechanism acts, said spring being inserted in a compressed condition.
The thrust of the spring and the friction generated between the first turn and the shoulder retain the mechanism within the box even if the box is open.
However, it is evident that this way of retaining the mechanism is not entirely safe, or rather, becomes safer as the compression of the spring increases.
However, the spring cannot be highly compressed, because during operation after assembly one would not have a sufficient further active stroke, also in view of its limited dimensions.
If a particularly hard spring were used to obviate this, one would no longer have a soft operation of the hinge during overtravel of the temples and one would again have more difficulty in preassembling the mechanism, since the spring must be compressed in order to be inserted.
Moreover, it should be noted that the use of springs that act by compression determines the dimensions of the hinges in which they are inserted.
The spring that acts by compression must in fact be inserted in a receptacle whose dimension must be at least equal to the length of the pack of the turns of the spring, plus a length equal to the stroke of the first articulation element of the hinge.
In view of the current requirements of the market, which requires ever smaller and less visible hinges, the dimensions of the spring are a constraint for the overall dimensions of the hinge.